Perhaps it would be better to explain why you even HAVE a "torque input" on a VFD.
The basic task that an AC induction motor performs is to provide torque at a given speed. The speed is based on the frequency applied, the torque is a function of the current drawn as a result of the LOAD put on the motor. More load = more current drawn = more torque produced, up to the limits of what the motor can do safely. You don't need a VFD to make that happen, that's inherent in how the motor works.
What the VFD does is to allow the motor to produce it's rated torque at any speed, by not just changing the frequency, but by changing it along WITH the voltage at a consistent rate. That's because the rated torque capability of the motor is based on maintaining a constant ratio of voltage and frequency, a V/Hz ratio. So for example a 460V 60Hz motor was designed around a V/Hz ratio of 7.67:1 so as long as you maintain that ratio, you get full torque capability of the motor. if I want to run at 60% speed, then 60% of 60Hz is 36Hz, so I need my voltage to be 36 x 7.67 = 276V RMS; that's what the VFD will do for me.
But if you get into ADVANCED use of VFDs, they can do what's called "Vector Control". This is based on the concept of the fact that during EACH sine wave going to the motor, FIRST, the power creates magnetic flux in the stator, THEN it produces torque. The V/Hz vector to produce current required to produce flux remains relatively constant regardless of speed, but the vector of V/Hz to produce TORQUE changes with loading. In a standard drive if I want more torque and just maintain that basic V/Hz ratio, I am over fluxing the motor at higher torque than necessary, so I am indirectly limiting the motor's ability to respond to changes in load without stressing it. So a Vector Drive is capable of separating those vectors in mid-cycle so that it only give is enough flux as it needs, leaving MORE available in the Torque current vector without adding unnecessary thermal stress. it also makes my motor respond faster and more accurately to changes in load.
So the last step is that now that I have that ability to control torque SEPARATELY from flux, it also gives me the ability to control torque regardless of speed. That's where the torque input come in to play. So lets say I have a winder application. If I am needing to have a constant tension in the payout and take up reels, I must maintain a VERY consistent torque on those reels. But at the same time, the speed is going to be changing on both sides, because the diameter will be changing. So by using a torque input, I can keep the torque perfectly controlled and pay no attention to the speed, because that is technically irrelevant to me now.
So bottom like, a torque input ONLY applies to doing torque control applications when the VFD is in Vector Control mode. If you are doing anything that is based on velocity, torque is consequential to loading and you don't need the input.
UNLESS... in your velocity control system, you have a MAXIMUM torque that your system can handle, then you MAY want to use that input as a to adjust the Torque Limit on the fly (you can usually set a fixed torque limit in programming, no need for the input).